title: Effects of repeated exposure to morphine in adolescent and adult male C57BL/6J mice: age-dependent differences in locomotor stimulation, sensitization, and body weight loss.
abstract: BACKGROUND
  Given evidence for age-related differences in the effects of drugs of abuse, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).
  
  
  OBJECTIVE
  This study compared the motor stimulating and ataxic effects of repeatedly administered morphine in adolescent, late adolescent, and adult mice.
  
  
  METHODS
  Mice were treated with saline or morphine (10-100 mg/kg, i.p.) once per day for 4 days, and morphine (3.2-56 mg/kg)-induced locomotion was assessed 3 days or 5 weeks later. Different mice were treated repeatedly with morphine and ataxia was measured.
  
  
  RESULTS
  Acute administration of morphine increased locomotion more in adolescents than in adults. Repeated morphine enhanced morphine-induced locomotion, assessed 3 days later, to a similar extent in each age group (minimum effective dose 17.8 mg/kg). This sensitization was still evident 5 weeks later when the adolescents had become adult, but was smaller and occurred at a higher dose (56 mg/kg). In animals treated repeatedly with morphine as adults, sensitization was no longer apparent 5 weeks later. Intermittent morphine was at least 10-fold less potent to produce body weight loss in adolescents than in adults. Repeated morphine did not alter morphine-induced ataxia at any age.
  
  
  CONCLUSIONS
  Compared with adults, adolescents were more sensitive to the acute locomotor stimulating effects of morphine and to its long-lasting locomotor sensitizing effects, consistent with overactivity of dopamine systems during adolescence. In contrast, adolescents were less sensitive than adults to body weight loss induced by intermittent morphine, an effect indicative of morphine withdrawal in adult rodents.
authors.affiliation: Departments of Psychiatry and Pharmacology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, Mail Code 7792, San Antonio, TX, 78229-3900, USA, koek@uthscsa.edu.
authors.forename: Wouter
authors.initials: W
authors.lastname: Koek
mesh_terms: D000375:Aging; D000818:Animals; D001259:Ataxia / Q000139:chemically induced / Q000503:physiopathology; D004305:Dose-Response Relationship, Drug; D008297:Male; D008810:Mice, Inbred C57BL; D009020:Morphine / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects* / Q000502:physiology*; D009294:Narcotics / Q000494:pharmacology*; D015431:Weight Loss / Q000187:drug effects* / Q000502:physiology*
chemical_list: D009294:Narcotics; D009020:Morphine